1. Field of the Invention
The subject invention relates to apparatus for generating control signals for positioning mechanical apparatus, and more particularly to an axis controller for positioning robotic members.
2. Description of Related Art
Commercial robot control algorithms are typically based on feedback of the error between the target position to which the robot is commanded to move and the actual axis position to which it moves. Tracking errors are inherent in pure feedback control systems since errors are required to produce a feedback driving signal.
Commercial robot control algorithms differ as to particulars, but share in common the fact that they are based on error (and derivative) feedback. Such error-driven controls yield large dynamic tracking errors, approximately proportional to the command velocity. Thus, traditional control algorithms yield a trade-off between speed and accuracy for each axis motor employed in the robot. Axis coordination is achieved by planning axis target paths that combine to generate the desired trajectory. Since axes will typically travel at different velocities, axis errors will not be coordinated. Additionally, robot arm coordination deteriorates with increasing speed. Accordingly, under the traditional control, an error "budget" is specified, which determines a maximum acceptable error, and robot speed is restricted so that the error remains within the predefined limits.
While every commercial robot is equipped with its own unique motion controller, one universal aspect of robot control is that the input to the motor which controls robot movement on an axis is a voltage. How this voltage is generated varies from robot to robot. Typically, the voltage is the output of a power amplifier, which serves to translate digital control commands into an analog voltage signal for the motor input. The specific choice of power amplifier, current or voltage mode, determines the nature of this translation, and therefore the nature of the digital command. Thus, the selection of a power amplifier is part of the design of the control algorithm itself.
A power amplifier is also a power regulator in that it functions to control an aspect of the motor power to a specified level. The particular aspect controlled depends on the specified motor characteristic which is fed back. A so-called current mode amplifier feeds back the motor armature current and varies the armature voltage in such a way as to attempt to make the armature current, i, equal to a command current, i.sub.c, calculated by the computer controller. This is the most common form of motion control amplifier.
For position control, there are several disadvantages of current mode amplifiers. One is that the motor command signal must be devoted primarily to acceleration as opposed to control at constant speed. Another is that current is more sensitive to torques, which are nonlinear and difficult to predict computationally.
A voltage mode amplifier feeds back and maintains the motor voltage proportional to the input voltage. There are several advantages of voltage mode amplifiers for position control. One is that the input is proportional to a motor voltage, which is proportional to motor speed. This proportionality makes voltage mode amplification inherently more stable than current modes for position control systems. Another is that the motor command signal range may be spent on control of constant velocity.
Another consideration common to design of commercial robot control algorithms is the level of smoothness of the target paths. Most target paths have continuous position and velocity profiles with jump discontinuities in the acceleration. Other types of target paths also include continuous acceleration profiles. The subject invention concerns command paths of the former type, i.e., those having jump discontinuities in the acceleration profile, but would also function with continuous acceleration. The control signal necessary for tracking of the target positions is provided to, for example, a power amplifier from a digital servo and data describing a desired target path crucial to operation of the robot. As noted, the tracking of the target positions contains inherent errors where traditional feedback algorithms are used.
Another approach to control algorithms is so-called feed forward or "open loop" control. In simulation and in theory, open loop control can track a smooth target path with zero error if the robot dynamics are known. Because of unknown loads, manufacturing variances, and other factors, however, these dynamics can never be known with absolute accuracy. Under pure open loop control, these uncertainties in the robot dynamics lead to position errors during the robot's motion.
Thus, both feedback and open loop control algorithms exhibit drawbacks which contribute to errors in positioning of robot elements.